Impact of Cooking and Rhizopus oligosporus Fermentation on Antinutritional Factors and Isoflavones in Soybeans

Abstract

The aim of this study was to investigate the effect of cooking and fungal fermentation during processing of fermented soybeans on antinutritional factors and isoflavones in soybeans. For this purpose, soybeans were cooked at 95 °C with a seed/water ratio of 1/3 for 10 to 60 min and fermented with the food fungus Rhizopus microsporus var. oligosporus. The kinetics of α-galactosides, phytic acid, and isoflavones were measured during cooking and fermentation. Diffusion in cooking water was also characterized and results for fungal fermentation were compared to a non-inoculated treatment. During cooking, α-galactosides, glycosides isoflavones, and in lower extent phytic acid decreased due to diffusion in cooking water and conversion of glycoside isoflavones in aglycone isoflavones. The decrease in α-galactosides was no longer significant or rather small after 30 min of cooking. Fermentation of 30-min cooked soybeans with R. oligosporus showed sporulation of the fungus and significantly reduced the total α-galactosides and glycosides isoflavones content of cooked soybeans, but did not affect phytic acid. Overall, the combination of cooking and fungal fermentation of soybeans significantly decreased α-galactosides and glycoside isoflavones by 69% and 80%, respectively, increased aglycone isoflavones up to 26.74 mg/100 g DM, but did not decrease significatively on phytic acid.